This invention relates generally to a method and apparatus for driving a non-impact type printer and more particularly, to a method and apparatus for driving an ink-on-demand type ink jet printer head operating with low voltage input. In a prior demand-type ink jet head driving method as disclosed in the U.S. Pat. No. 4,161,670, a voltage of a polarity opposite to the polarity of the polarization voltage of a piezoelectric element is applied to the element to maintain the wall of a pressure chamber in such a condition that the volume of the pressure chamber is increased. After a predetermined period of time, the polarity of the voltage applied to the piezoelectric element is inverted to thereby reduce the previously enlarged volume of the pressure chamber. Thereby an ink droplet is ejected from the nozzle of the printer head. A voltage transformer is employed to invert the polarity of the applied voltage. A secondary inductance of the voltage transformer forms an oscillating circuit with the capacitance of the piezoelectric element. The resonant frequency of the oscillating circuit is set equal to the mechanical resonant frequency of the column of ink and the period of primary current impact is equal to half of the period of the resonant frequency.
For implementing such a driving method of the prior art, a separate voltage transformer and control circuit are necessary for each nozzle. Therefore, in the case of a multi-nozzle type ink jet head, the total cost of the assembly is quite high because it is necessary to provide as many voltage transformers and control circuits are there are nozzles.
However, to reach the maximum efficiency for which the highest velocity of ink droplets is obtained with the lowest voltage, the period of the primary current impact should not be the same as a half period of the resonant frequency of the column of ink as in the prior art, for the following reasons. The oscillation of the ink column is a transient response to the primary current impact of the voltage transformer in a system composed of a wall of the pressure chamber, the piezoelectric element and the ink itself. Accordingly, the mechanical/hydraulic oscillation is a damped oscillation involving a phase lag related to the driving waveform applied to the piezoelectric element. Therefore, the instantaneous time when the increased volume of the pressure chamber is decreased by changing the polarity of the voltage applied to the piezoelectric element, should be selected to occur in correspondence with the phase of the damped mechanical hydraulic oscillation, taking account of the phase lag of the column of ink to obtain the above described maximum efficiency. In other words, when the period of the primary current impact of the voltage transducer is not set equal to a half period of the resonant frequency of the column of ink, but is set to coincide with an optimum phase of the actual damping oscillation of the column of ink in the pressure chamber and the nozzle, the ink droplets can be jetted with a low voltage.
It has been confirmed through experiments that the period of the above described current impact for maximum efficiency of operation is longer than the half period of the natural frequency of the column of ink, chamber and piezoelectric element. With this method, the magnitude of voltage which is applied to the piezoelectric element to modify the volume of the pressure chamber is reduced. Therefore, as the applied voltage decreases, the piezoelectric element is better protected from depolarization.
What is needed is a method and apparatus for driving an ink jet printer head having high efficiency and using low voltage.